1. Field of the Invention
This invention relates generally to a magnetoresistive random access memory (MRAM) cell formed in a magnetic tunneling junction (MTJ) configuration and particularly to a form of such cell in which the read and write functions of the cell are assigned to different free layers.
2. Description of the Related Art
The magnetic tunneling junction (MTJ) device, is a form of giant magnetoresistive (GMR) device in which the relative orientation of uni-directional magnetic moments in parallel, vertically separated upper and lower magnetized layers, controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those layers. When injected electrons pass through the upper layer they are spin polarized by interaction with the magnetic moment of that layer. The probability of such an electron then tunneling through the intervening tunneling barrier layer into the lower layer then depends on the availability of states within the lower electrode that the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower electrode. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability times number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. The MTJ device can therefore be viewed as a kind of variable resistor, since different relative orientations of the magnetic moments will change the magnitude of a current passing through the device.
FIG. 1 is a schematic cross-sectional view of a typical MTJ layer structure formed in what is called a spin-filter configuration. In this particular form, the lower one of the two magnetized layers. now called a pinned layer, has the direction of its magnetic moment fixed in direction, while the magnetic moment of the upper or free layer remains free to move in response to external stimuli. Looking from the bottom up, the layer configuration includes of a seed layer (5), that is used as a foundation on which to form successive overlayers. A layer of antiferromagnetic material. the AFM layer (10). is formed on the seed layer and will be used to pin the magnetic moment of the pinned layer by a form of magnetic coupling called exchange coupling. The lower, pinned layer (20) is a layer of ferromagnetic material formed on the AFM layer, or it can be a pair of ferromagnetic layers separated by a non-magnetic coupling layer. The tunneling barrier layer or junction layer (30) is then formed on the pinned layer, typically by first forming a layer of a metal such as aluminum (or magnesium) and then subjecting the aluminum to oxidation. The free layer (40) is a ferromagnetic layer that is then formed on the junction layer. Finally, a protective capping layer (50) is formed on the free layer.
If the magnetization of the free layer is allowed to move continuously. as when it is acted on by a continuously varying external magnetic field, the MTJ device can be used as a read-head for sensing magnetic field variations produced by moving magnetically recorded media. If the magnetization of the free layer is constrained to move in only two directions, eg. parallel to or antiparallel to the magnetization of the pinned layer, then the MTJ device can be used as a memory device, called a magnetic random access memory device or MRAM. When used as an MRAM, the MTJ device provides only two resistance values, maximum resistance in the antiparallel orientations of the free and pinned layer magnetizations and minimum resistance in their parallel orientation. Thus, when the device in one of its two resistance states it can be said to store a logical zero or one. By sensing the resistance state, which requires the passage of a current through the device, the device is “read,” and by changing the resistance state, which requires an external magnetic field produced adjacent current-carrying conductors, the device is written upon. As noted, the reading and writing of such a device is accomplished by its interaction with the magnetic fields of current carrying lines, called word lines and bit lines, that are vertically separated and typically pass above and below the MTJ device in mutually perpendicular directions. During quiescent states of the cell, when there are neither currents nor fields interacting with it, the cell stores information, meaning that the magnetic orientation of its free layer remains fixed.
In order that the read, write and storage operations be performed efficiently, it is desired that the magnetization of the free layer remains preferentially aligned or anti-aligned with the magnetization of the pinned layer when the cell is in a quiescent state, that is, when magnetic fields and/or currents are not being applied to it. The ability of the cell to maintain this alignment is what makes it an effective storage device. FIG. 1 shows an arrow (25) representing the fixed magnetic moment of the pinned layer and two arrows (45) and (47), representing the two possible directions of the free layer magnetic moment corresponding to low (45) and high (47) resistance of the cell.
In order to make the free layer retain its magnetization direction when the cell is quiescent, the free layer is provided with a degree of magnetic anisotropy, meaning that its magnetization prefers a particular direction. Typically, this anisotropy is “shape anisotropy,” which is the result of making the cell longer in one direction than another, for example, by making the horizontal cross-sectional shape elliptical rather than circular. However, if the free layer is given a great deal of magnetic anisotropy, it will hold its magnetization very effectively but it will be increasingly difficult to change that direction when it is required to write upon the cell. Thus, there is a trade-off between the storage capability of a cell and the energy that must be expended to write upon that cell.
There are several factors that affect the performance of an MRAM cell having the configuration described above.
a) The magnetic coupling between the free and pinned layer is affected by the roughness of the oxide layer between them. The role of surface roughness in coupling the magnetization of the free and pinned layers is often called the “orange peel” effect (or Neel coupling) and it plays a role in establishing the switching threshold of a cell.
b) The uncompensated magnetic poles at the edges of the pinned layer (ie., the divergence of the magnetization vector at the edges of the layer) produces a bias against the switching of the free layer and, thereby, renders the switching threshold non-uniform among an array of cells.
c) For reliable switching behavior, the ferromagnetic free layer is generally limited to being formed of materials with small intrinsic coercivity. This makes it disadvantageous to use certain materials that produce a high magnetoresistive (MR) ratio (Dr/r), such as CoFeB and CoFe, whose high concentration of Fe produce good MR ratios, but which have generally higher intrinsic coercivities causing large switching field variations. Note that a high MR ratio produces a large difference between high and low resistance states which are more easily sensed in the read process.
Prior art teachings can be found that attempt to address at least certain of these problems. Drewes, (U.S. Pat. No. 6,900,489 B2) teaches a method of reducing the adverse effects of Neel coupling, which is the magnetostatic coupling (ie., the coupling due to edge charges) between the free and pinned layers across the barrier layer. The method of Drewes is to form an additional ferromagnetic layer on the antiferromagnetic pinning layer (layer (10) in FIG. 1 of this application). Drewes asserts that such an additional ferromagnetic layer will reduce the amount of external field required to produce switching of the cell.
Parkin et al. (U.S. Pat. No. 6,166,948) also addresses the adverse effects of magnetostatic interactions by forming a free layer of two oppositely magnetized ferromagnetic layers coupled across a spacer layer. This laminated free layer therefore has closed magnetization loops at its edges and, consequently, is free of uncompensated magnetic poles at those edges (zero divergence of the magnetization), reducing the effects of magnetostatic interaction.
Worledge (U.S. Pat. No. 6,833,573 B3) teaches a method in which a magnetic anisotropy is provided by means of a memory cell having a curved magnetic region.
Accordingly, it is the object of the present invention to provide a new MRAM design wherein the tradeoff between anisotropy and ease of switching does not exist, because the MRAM cell has a divided free layer, producing two separate parts: a read-sensing free layer having little or no anisotropy and an information storage free layer which has the necessary anisotropy to provide a field that aligns the read-sensing layer.